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United States Patent |
5,568,208
|
Van de Velde
|
October 22, 1996
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Modified scanning laser opthalmoscope for psychophysical applications
Abstract
A modified scanning laser ophthalmoscope expands the range of clinical
applications of the conventional scanning laser ophthalmoscope, being able
of presenting the scanning laser raster with graphics to the retina and
simultaneously allowing the observation of the anterior segment on the
display monitor. The device, including a beamsplitter, infrared
lightsource, scanning laser ophthalmoscope, CCD camera, and optical
filters, determines unambiguously in real-time the entrance pupil of the
Maxwellian view scanning laser ophthalmoscope. The location of the
entrance pupil and stimulus position on the retina can be moved
independently.
Inventors:
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Van de Velde; Frans J. (2 Hawthorne Pl. 15-O, Boston, MA 02114)
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Appl. No.:
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207385 |
Filed:
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March 8, 1994 |
Current U.S. Class: |
351/221; 351/205 |
Intern'l Class: |
A61B 003/10 |
Field of Search: |
351/200,205,206,208,221
354/62
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References Cited
U.S. Patent Documents
4213678 | Jul., 1980 | Pomerantzeff et al. | 351/206.
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5308919 | May., 1994 | Minnich | 351/221.
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Other References
Nasemann and Burk, Scanning Laser Ophthalmoscopy . . . ISBN 3-928036-01-7
Chapter 1 and 2, pp. 23 to 46 (1990).
Wyszecki and Stiles, Color Science: concepts and . . . ISBN 0-471-02106-7
Chapter 5.11 Stiles-Crawford effect pp. 424 to 429; Chapter 5.15.1/2/3
Maxwellian view, pp. 478-485 (1982).
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Primary Examiner: Sikes; William L.
Assistant Examiner: Mai; Huy
Claims
I claim:
1. A scanning laser ophthalmoscope with Maxwellian view control for imaging
and psychophysics, comprising of:
(A) a scanning laser ophthalmoscope, using Maxwellian view illumination and
having (1) illuminating means including at least one wavelength in the
visible range and a wavelength in the infra-red range of the spectrum for
visualizing the posterior segment or retina of the eye and psychophysical
testing, said scanning laser ophthalmoscope also provided with (2) a
modulating means for creating psychophysical stimuli in the visible laser
raster of said scanning laser ophthalmoscope and (3) an electronic means
for generating video and common synchronization signal;
(B) a second imaging device, using free Newtonian viewing, for visualizing
the anterior segment of the eye and the reflection or backscatter of the
coincident illuminating means of said scanning laser ophthalmoscope in the
pupillary area and on the iris, simultaneously with the observation of the
posterior segment of the eye with said scanning laser ophthalmoscope, said
imaging device allowing this simultaneous viewing of both the anterior and
posterior segment of the eye without affecting adversely the quality of
the retinal image, and said imaging device having focusing means to
document the extent and location of the Maxwellian view in the pupillary
area using the anterior segment as a fiducial landmark;
(C) an imaging board with means for overlay graphics, CPU and monitor, for
controlling modulating means in said scanning laser ophthalmoscope and
displaying simultaneously the extent and precise location of the
Maxwellian view in the pupillary area of the anterior segment together
with an image of the retina and the characteristics of any psychophysical
stimulus projected onto the retina using modulating means of said scanning
laser ophthalmoscope;
whereby the simultaneous availability of both images of the anterior and
posterior segment of the eye enable said scanning laser ophthalmoscope to
freely move, maintain and adjust for focusing the Maxwellian view
illumination in the pupillary area of the eye, simultaneously document the
unambiguous position of this Maxwellian view on an image of the pupillary
area of the eye and simultaneously document the retinal image with the
position of any stimulus of which the location can be selected
independently from the selection of a location in the pupillary area for
the Maxwellian view illumination, thereby allowing to measure the
Stiles-Crawford effect on imaging and psychophysics for selected
Maxwellian view illumination and selected retinal area.
2. The scanning laser ophthalmoscope with Maxwellian view control according
to claim 1 further including the improvement of a beamsplitter for
changing the optical path from the anterior segment of the eye to said
imaging device, said beamsplitter introduced in such manner as to allow
said scanning laser ophthalmoscope to retain optimal frontal position with
regard to the eye and said beamsplitter not affecting the quality of the
retinal image and anterior segment;
whereby combination of said scanning laser ophthalmoscope with said imaging
device and with said beamsplitter optimizes the viewing of the anterior
segment of the eye by allowing less restrictions imposed on size and
position of said imaging device and by providing a frontal viewing of the
pupillary area, thereby facilitating and making more precise the moving,
maintaining and documenting the size and location of the Maxwellian view
illumination of said scanning laser ophthalmoscope ophthalmoscope in the
pupillary area.
3. The scanning laser ophthalmoscope with Maxwellian view control according
to claim 1 further comprising second infra-red light illuminating means,
using a third wavelength, for the anterior segment of the eye, said second
infra-red light illuminating means adjusted in intensity, properly
oriented and using such wavelength that no adverse effect is created for
observing the retina and projecting psychophysical stimuli with said
scanning laser ophthalmoscope;
whereby an improved visualization of fiducial landmarks is obtained around
the pupillary area in the anterior segment for the localization of the
reflection and backscatter of illuminating means of said scanning laser
ophthalmoscope in the pupillary area.
4. A method for controlling the Maxwellian view illumination of a scanning
laser ophthalmoscope during imaging and psychophysics comprising the steps
of:
(A) visualizing the posterior segment or retina of the eye and
psychophysical testing with a scanning laser ophthalmoscope, using
Maxwellian view illumination with illuminating means comprising at least
one wavelength in the visible range and a wavelength in the infra-red
range of the spectrum, said scanning laser ophthalmoscope also providing
psychophysical stimuli in the visible laser raster of said scanning laser
ophthalmoscope with a modulating means and providing common
synchronization of all video signals using electronic means;
(B) visualizing the anterior segment of the eye with the reflection and
backscatter of the coincident illuminating means of said scanning laser
ophthalmoscope using a second imaging device and free Newtonian viewing,
simultaneously with the observation of the posterior segment of the eye
with said scanning laser ophthalmoscope, said imaging device permitting
simultaneous viewing of both the anterior and posterior segment of the eye
without restricting the visualization of the retinal image, and said
imaging device having a focusing means for documenting the extent and
location of the Maxwellian view illumination in the pupillary area using
the anterior segment as a fiducial landmark;
(C) controlling modulating means in said scanning laser ophthalmoscope with
an imaging board permitting overlay graphics, CPU and monitor, and
displaying simultaneously the extent and location of the Maxwellian view
in the pupillary area of the anterior segment together with an image of
the retina and the overlay of any psychophysical stimulus projected onto
the retina using modulating means of said scanning laser ophthalmoscope;
whereby the availability of both images of the anterior and posterior
segment of the eye allows said scanning laser ophthalmoscope to freely
move, maintain and adjust for focusing the Maxwellian view illumination in
the pupillary area of the eye, simultaneously document the unambiguous
position of this Maxwellian view on an image of the pupillary area of the
eye and simultaneously document the retinal image with the overlay of any
stimulus of which the location can be selected independently from the
selection of a location of the Maxwellian view illumination, thereby
allowing to demonstrate the Stiles-Crawford effect on imaging and
psychophysics for combination of selected Maxwellian view illumination and
selected retinal area.
5. The method for controlling the Maxwellian view illumination of a
scanning laser ophthalmoscope according to claim 4 further including the
improvement of the use of a beamsplitter for changing the optical path
from the anterior segment of the eye to said imaging device, said
beamsplitter introduced in such manner as to allow said scanning laser
ophthalmoscope to retain optimal frontal position with regard to the eye
and said beamsplitter not affecting the quality of the retinal image and
anterior segment;
whereby use of said beamsplitter optimizes the viewing of the anterior
segment of the eye by allowing less restrictions on position and size of
said imaging device and by providing a frontal viewing of the pupillary
area, thereby facilitating and making more precise the moving, maintaining
and documenting the size and location of the Maxwellian view illumination
of said scanning laser ophthalmoscope in the pupillary area.
6. The method for controlling the Maxwellian view illumination of a
scanning laser ophthalmoscope according to claim 4 further including the
use of a second infra-red light of a third wavelength for illuminating the
anterior segment of the eye, said second infra-red light adjusted in
intensity, properly oriented and of such wavelength that no adverse effect
is created for observing the retina and for projecting psychophysical
stimuli onto the retina with said scanning laser ophthalmoscope;
whereby additional visualization of fiducial landmarks is obtained in the
anterior segment of the eye for the localization of the reflection and
backscatter of illuminating means of said scanning laser ophthalmoscope.
Description
BACKGROUND-CROSS REFERENCE TO RELATED APPLICATION
The invention uses the scanning laser ophthalmoscope proper of co-pending
application, Ser. No. 08/178,777, filed 1994 Jan. 7.
1. Background-Field of Invention
This invention relates generally to instruments for examining the eye and
specifically to an electro-optical ophthalmoscope for providing
simultaneously a precise visual representative of the eye fundus, anterior
eye segment, and eye functioning on a display monitor.
2. Background-Description of Prior Art
The ophthalmoscope is well known as an important aid for studying and
examining the eye, and in particular, the fundus of the eye. As a result
of great interest in preserving man's eyesight, ophthalmoscopes of various
constructions have been built and used. The latest version of the
ophthalmoscope, a scanning laser ophthalmoscope, is particularly appealing
because of its unique capability of combining the visualization of the
retina or eye fundus with certain psychophysical and electrophysiological
testing procedures, used in studying the subjective or objective
functioning of the visual pathways, from the retina to the brain cortex.
With the scanning laser ophthalmoscope, a unique, precise correlation
between retinal anatomy and function is established. Many different
stimuli that are used in visual psychophysics, can be projected onto the
fundus with the help of the scanning laser ophthalmoscope. Computer red
overlay graphics are then used to display the stimulus characteristics
such as size, location, and intensity on the fundus image in real-time.
Detailed functional mapping of the fundus is thereby possible. Such
functional mapping that is currently possible emulates classic Goldmann
kinetic perimetry and automated static perimetry under light-adapted
testing conditions. Until the invention, the scanning laser
ophthalmoscope, has been limited to the examination of the posterior
segment, excluding simultaneous imaging of both the retina, with
presentation of psychophysical stimuli, and iris plane . Visualization of
the anterior segment is however important because it allows an unambiguous
observation of the entrance pupil of the Maxwellian view illumination used
by the scanning laser ophthalmoscope. This is very significant since light
entering near the center of the pupil is more efficient in eliciting a
visual response than is light entering peripheral regions of the pupil.
Not knowing the entrance pupil therefore precludes such testing as
dark-adaptation, measuring dark-adapted thresholds and the Stiles-Crawford
functions with the scanning laser ophthalmoscope.
Furthermore, eye movements and changes in the subject's fixation have
hitherto limited the accuracy and ease of performing high resolution
perimetry or microperimetry on the fundus. In practice, repeat trial and
error presentation of one stimulus to one specific retinal location, using
fiducial landmarks as a guide, can result in a waste of time, discomfort,
decreased performance and limited information from the testing. The
entrance pupil of the Maxwellian view is even more difficult to control
manually since no reliable fiducial landmarks are available in the
pupillary area.
OBJECTS AND ADVANTAGES
The principal objects of this invention are therefore to provide a modified
scanning laser ophthalmoscope having the capability of presenting stimuli
or graphics to the retina of one eye and to record simultaneously the
entrance pupil coordinates of the Maxwellian view together with a view of
the retina and stimulus or graphics overlay on a display monitor. The
entrance pupil and stimulus location on the retina can be selected
independently from each other and are subject to a passive, such as a
bite-bar and trial-error-repeat stimulation, or active, such as fundus and
pupil tracking, feedback mechanism to ensure their position regardless of
eye movements or fixation shifts. With other words, it is possible now to
take into account variations in visual function that are observed, not
only when stimuli are imaged on different retinal areas, but also when
stimuli enter the eye through different parts of the eye pupil on a
monitor on the same very specific retinal area, displayed on a monitor.
Several diverse objects and advantages of the device can be envisioned. In
imaging, precise positioning of the entry pupil facilitates normal and
off-axis dark-field viewing of the retina and brings an important variable
in image-based reflectometry or densitometry measurements under control.
As explained before, the entrance pupil of the Maxwellian view
illumination is also a very important variable in functional testing of
the eye, either subjective psychophysics or objective electrophysiology.
Knowing its position during testing, allows measuring dark-adapted
thresholds, Stiles-Crawford I and II effects, the Campbell effect, and
dark-adaptation constants for specific areas of the fundus that can be
visualized on the monitor. Active feedback tracking algorithms that
document and stabilize the Maxwellian view and stimulus position on the
retina are also the final step towards fully automated static
microperimetry strategies. Stimulus presentations for example can be
automatically repeated after the subject blinks or has repositioned in the
chinrest.
It has been demonstrated that dark-adapted thresholds correlate with the
amount of photopigment that is present in the retina. Dark-adaptation
constants parallel the speed of replenishing of photopigment after
bleaching. Stiles-Crawford functions reveal the general orientation and
degree of alignment of the photoreceptors. Abnormalities in these
parameters reveal damage to the retinal pigment epithelium-photoreceptor
complex in the retina. This damage may be caused for example by a failure
to maintain a critical oxygen gradient across the complex and as a result
a reduced photopigment regeneration in Wald's cycle. It has been
demonstrated that the abnormalities described above are relevant to visual
functioning when macular disease, in particular the early stages of both
types of age related macular degeneration, the leading cause of legal
blindness in the United States, are present. Detailed functional retinal
maps of the Stiles-Crawford functions, dark-adaptation and dark-adapted
visual thresholds will very likely lead to insight in the physiopathology
of the disease. They will prove to be useful in planning laser treatment
indications, strategies, and follow-up. An example of such laser treatment
could be the generation of oxygen windows through multiple mild
chorioretinal burns. Further objects and advantages of the invention will
become apparent from a consideration of the drawings and ensuing
description.
DESCRIPTION OF THE DRAWINGS
Features and advantages of the invention will appear from the following
description of preferred embodiments of the invention, taken together with
the drawings in which:
FIG. 1 is a diagrammatic representation, illustrating the general mode of
operation of the modified scanning laser ophthalmoscope; and
FIG. 2 is a perspective view of the modified scanning laser ophthalmoscope.
The perspective view shows the spatial relationship between subject,
scanning laser ophthalmoscope proper, and the electro-optical pathway
including CCD camera, beamsplitter and anterior segment illumination.
REFERENCE NUMERALS IN DRAWINGS
10 Prefocussed narrow Gaussian beam of laser light
12 Posterior pole of the fundus, retina
14 Scanners, including polygon and galvanometer
16 Optical entrance pupil of the Maxwellian view in the iris plane
18 Reflected and backscattered light from the eye
20 Beam separator
22 Pinhole at the retinal conjugate plane
24 Avalanche photodiode
26 Video display monitor
28 Diode infra-red 780 nm laser
30 Amplitude modulation of diode laser
32 He-Ne red 632 nm laser
34 Pair of adjustable linear polarizers
36 Graphics on the retina, visible as overlays
38 Acousto-optic modulator
40 Electronic circuitry of scanning laser ophthalmoscope
42 Distribution of common synchronization to different components
44 Encasement
46 Beamsplitter
48 Superluminescent 880 nm LED
50 Nose, cheeks and eyebrows
52 Cover of scanning laser ophthalmoscope
54 Slanted window of scanning laser ophthalmoscope with diaphragm
56 CCD videocamera with objective
58 Bite bar
60 Chinrest
62 Vertical movement mechanism using stepper motor
64 Horizontal two dimensional movement mechanism using stepper motors
66 486/33 mHz CPU with overlay frame grabber board
68 Overlay frame grabber board
70 Entrance pupil laser beam as overlay on image of anatomical pupil
72 Partial infrared barrier filter
74 Additional graded neutral density filter and diaphragm
DESCRIPTION AND OPERATION OF AN EMBODIMENT-FIG. 1,2
A typical embodiment of the modified scanning laser ophthalmoscope is
illustrated by FIG. 1 and 2. The principles of scanning laser
ophthalmoscopy are described in detail in the prior art. Features relevant
to the invention are further discussed.
THE SCANNING LASER OPHTHALMOSCOPE WITHIN THE MICROPERIMETER
A prefocussed narrow Gaussian beam of laser light 10, typically 12.mu. in
diameter at the retinal plane, is scanned over the posterior pole of the
eye 12 in a sawtooth manner with the help of scanning mirrors, currently a
polygon and galvanometer 14. Both fast horizontal 15 KHz and slower
vertical 60 Hz deflections of the flying laser spot are at standard video
RS-170 rates with blanking intervals and create the rectangular laser beam
raster that is seen by the subject. A Maxwellian view 16 is used in the
illuminating portion of the scanning laser ophthalmoscope: the pivot point
of the scanning laser beam is actually a tiny three dimensional volume
optimally situated in the iris plane with an average waist of less than 1
mm. Typically a rectangular area of approximately 0.5 cm.sup.2 on the
retina is illuminated. This corresponds to a field of view of 40 degrees
in diagonal or 32.7 degrees horizontally by 23.4 degrees vertically. The
field of view can be changed with electronic or optical adjustments of the
optical path. It is important to understand that the subject will not see
a flying spot but rather a rectangle filled with thin horizontal stripes
because of the temporal summation characteristics of the visual system.
The reflected and backscattered light 18 of the eye, now filling the
pupil, is descanned over the same mirrors, separated from the illuminating
beam and passed through a pinhole 22 at the retinal conjugate plane before
reaching a fast and sensitive avalanche photodiode 24. This confocal
detection method is essential for obtaining high contrast specular
pictures of the retina with infra-red illumination, by eliminating stray
light at the pinhole. Non-confocal viewing modalities, also called
dark-field, indirect or Tyndall imaging, are obtained by the insertion of
a central stop instead of the pinhole 22 or off-axis illumination of the
retina. The amount of light on the photodetector is translated into a
voltage that modulates the intensity of an electron beam on the visual
display cathode ray tube monitor 26. The electron beam moves
synchronically with the scanning laser beam and a real-time video image of
the fundus is likewise created on the display monitor. Two laser sources
are aligned to illuminate the retina. The two lasers serve a different
purpose. A high intensity diode infra-red 780 nm laser 28, electrically
modulated and vertically polarized, is nearly invisible to the subject. It
produces the retinal image on the display monitor. A superimposed low
intensity He-Ne red 632.8 nm red laser 32, modulated with a pair of
adjustable linear polarizers 34 and horizontally polarized, is visible to
the eye. It is used to draw psychophysical stimuli 36 in the laser raster
for projection onto the retina. These stimuli are created by amplitude
modulation of the laserbeam at video rates as the red light passes through
an acousto-optic modulator 38. The acousto-optic modulator is driven by a
standard video source, usually a computer overlay frame grabber card that
contains the graphics information, and is genlocked to the crystal clock
of the electronic circuitry 40. Master timing signals are derived from the
spinning polygon. It is important to understand the reason for using two
different lasers. The scanning laser ophthalmoscope is very light
efficient: about three orders of magnitude less light is necessary to
visualize the fundus when compared with conventional ophthalmoscopes.
However this light is still orders of magnitude the amount used for
typical psychophysical testing. The problem is solved by using a 780 nm
laser with sufficiently high output and for which the silicon detector of
the scanning laser ophthalmoscope, but not the eye, is most sensitive, in
combination with a low intensity 632 nm laser, for which the human eye is
sensitive but insufficient for visualizing the fundus. This explains also
why the stimuli which are perceived by the subject are usually not visible
in the retinal picture, unless very bright. The exact position and
characteristics of the stimuli can however be shown in real-time on the
retinal image with the help of computer overlays as all image video out of
the scanning laser ophthalmoscope, scanners, and graphics video into the
acousto-optic modulator are synchronized to the same crystal clock 42. The
scanning laser ophthalmoscope preferably uses achromatic refracting
surfaces, such as mirrors and a polygon instead of lenses and
acousto-optic deflectors, to prevent chromatic aberrations when combining
two different wavelengths. Specific infra-red light retinal scatter and
penetration, absorption, and reflection characteristics combine in a very
complex fashion with the directional selectivity of light input, output
and prefocussing of the laserbeam to visualize early and subtle changes in
the photoreceptor--retinal pigment epithelium complex. These changes are
hardly visible at times with conventional illumination techniques. The
above mentioned variables, some of which are wavelength specific, together
with video gain level, laser intensity, DC coupling within the monitor,
and degree of pinhole or central stop determine the image appearance. High
quality imaging is a prerequisite for Fundus related psychophysical
testing. The scanning laser ophthalmoscope used in the modified scanning
laser ophthalmoscope allows real-time and precise control of all the
variables mentioned in this paragraph.
LASERSOURCES AND MODULATION OPTIONS
The diode 780 nm laser 28 can be replaced by a diode laser of longer
wavelength, for example 904 nm. For every 10 nm increase in wavelength of
the diode laser beyond 670 nm, the efficiency for stimulating the retina
will be reduced to one half. This is useful in making the infra-red
background illumination for visualizing the retina on the display monitor
even less visible to the subject. Surface-emitting quantum-well laser
diodes are of increasing interest, and offer the advantages of high
packing densities on a wafer scale. An array of up to a million tiny
individually modulated cylindrical In.sub.0.2 Ga.sub.0.8 As
surface-emitting quantum-well laser diodes with lasing wavelengths in the
vicinity of 970 nm and shorter can substitute the traditional laser
sources and scanners of a scanning laser ophthalmoscope. This will render
the device more compact, less noisy, and less susceptible to mechanical
wear and tear.
The 632.8 nm He-Ne laser 32 has been incorporated in the scanning laser
ophthalmoscope because of its compact and sturdy design. It is however
also the wavelength of choice for generating graphics and stimuli. Reasons
for this preference are the maximal transmittance and minimal scatter
within the transparent media of the eye, a minimal interference with the
xanthophyll and hemoglobin pigments, and a monophasic cone response when
compared with shorter visible wavelengths such as 488-514 nm blue/green
argon. These advantages persist if a longer visible wavelength were
chosen, e.g. a diode laser at 650 nm. In traditional Goldmann and
automated perimeters, stimuli and background illumination are provided by
different optical pathways, the lightsource however being the same. The
advantages of this configuration are twofold. First, the background and
stimulus fluctuate in the same amount as the lightsource is varying in
intensity. Second, additional separate modulation of background and
stimulus is possible with a neutral density filter and thereby a minimal
or zero background with a maximum stimulus as permitted by the light
source, is possible. Unlike the Goldmann and automated perimeters, stimuli
and background illumination are always provided by the same optical path
and light source in the scanning laser ophthalmoscope and cannot be
further modulated with e.g. a separate neutral density filter for each
channel. A truly zero background is not possible with either electrical
amplitude modulation or acousto-optic modulation and the ratio between
stimulus and background has a maximum limit. For acousto-optic modulation,
the theoretical limit is 700, 300 is the practical and desired limit but
very often only 100 is obtained. Incorporating the 650 nm diode laser can
expand the dynamic range of stimulus to background illumination intensity
by combining in parallel the electrical high-frequency amplitude
modulation of the diode with the acousto-optic modulation described
before. For this purpose the blue and red output of the overlay frame
grabber board are used. The circuitry is easily constructed by s/he who is
skilled in the art of electronics.
The measurement of dark-adapted thresholds, dark-adaptation and
Stiles-Crawford functions require a minimal background and sufficiently
intense maximum stimulus. A dynamic range of maximum stimulus intensity to
background intensity of 2.5 log units is acceptable. As mentioned before,
the acousto-optic modulator is driven by a standard video source, a
computer overlay frame grabber card that contains the graphics
information, and is genlocked to the crystal clock of the electronic
circuitry. A typical example of such a video card is the FG 100-AT or more
recently introduced, the OFG card, both available from Imaging Technology
Inc., Bedford Mass. The OFG card is also the essential hardware for
alignment and tracking of the pupil or fundus landmarks as described
below. Two OFG cards, which are I/O mapped can reside in one CPU,
typically equipped with a Intel 486/33-66 MHz microprocessor. The OFG
board has to provide three basic functions for realizing the different
psychophysical testing and imaging procedures with the scanning laser
ophthalmoscope: generation of 8 bits graphics, frame grabbing and display
of overlays. The different testing algorithms are constructed from an
appropriate sequence of software routines by s/he who is skilled in the
programming art. The library of basic software routines is, as usual,
provided by the board manufacturer. Specific bitlevels of the graphics
board, out of 256 possibilities, are combined with appropriate neutral
density filter settings placed in various positions for approximate and
fine tuning of laser light intensities. Using a combined neutral-density &
beam modulation technique, we can obtain a uniform 0.1 log U intensity
scale. This scale is often used in psychophysics. Individual bitlevels are
translated into corresponding intensity levels by converting each bitlevel
to an analog voltage with the D/A of the graphics board. This output
voltage is then amplified and off-set by the AOM driver electronic circuit
to obtain suitable voltages for modulating the amplitude of a
radio-frequency carrier signal. The piezo-electric transducer of the
acousto-optic-modulator is driven by this signal. Acoustic standing waves
are created by the piezo-electric transducer. The atoms of the crystal
then behave as a diffraction grating for the transversing laser beam. The
amount of light passing through the diffraction grating as a first-order
beam at the Bragg angle, defined by the frequency of the radio signal, is
directly proportional to the amplitude of the radio-frequency signal. The
time required for the AOM to adjust the intensity after changing bitlevels
is called the AOM delay. The delay has to be accounted for in calibrating
the overlay graphics. Inherent fluctuations in the electronics of D/A
converter, AOM driver, and laser output will not affect the ratio of light
intensities corresponding to any two bitlevels. Short term and long term
variations of individual bitlevel intensities exist. Short term
fluctuations are irrelevant for psychophysics since they are usually
smaller than the psychophysical uncertainty and long term variations are
neutralized with the help of a radiometer. Minimal spatial variations of
light intensity also exist within the laser beam raster. They are
neutralized with either software or a graded neutral density filter. Also
a variability in local laser raster geometry has to be taken into account.
A pincushion or trapezoid deformity of the near rectangular raster is
caused by the internal optical configuration of the scanning laser
ophthalmoscope. External angular dimensions of graphics are determined
with the arctan formula. On the retina, equivalent linear distances are
calculated using the standard observer's eye of LeGrand with an effective
optical radius of 16.7 mm. 300.mu. on the retina equals 60 external
minutes of arc. In practice, an uncertainty of about 10% in absolute size
is expected because of the metric nonlinearities described and the
variability in individual eye optics.
In summary, The laser raster itself is a very unusual for presenting
graphics to the retina, graphics are presented in multiples of 33 ms, the
time interval to draw one complete video frame on the retina. Each video
frame consists of two interlaced video fields of 16.7 ms and 256 lines,
the video fields may overlap during eye movements. Within each video field
the graphics is composed of discrete pixels and every pixel is illuminated
with a Gaussian beam profile in Maxwellian view for only 77 nanoseconds
each. This is in sharp constrast with the smooth delivery of photons to
the retina in conventional Ganzfeld, Newtonian illumination of standard
perimeters. However the same number of photons arrive on the retina in
both illumination systems, but with a completely different spatial and
temporal distribution. Under physiologic testing conditions, it has been
demonstrated that the laws of Bloch, Bunsen-Roscoe, Ricco, Weber-Fechner,
Rose-DeVries, and the power law of psychophysics are applicable and
equivalent in both illumination systems.
EXTERNAL APPEARANCE OF THE MODIFIED SCANNING LASER OPHTHALMOSCOPIC
The configuration of FIG. 2 is for use with the Rodenstock Scanning Laser
Ophthalmoscope 101 or 102 (Munich, Germany). This configuration is readily
adapted for other embodiments of the scanning laser ophthalmoscope by s/he
who is skilled in the art. The principal new optical components of the
modified scanning laser ophthalmoscope are contained in a moulded and
closed encasement 44 for dust protection. These components include the
beamsplitter 46, optical filters, and illumination source 48 for the
anterior segment. The encasement is painted matt black on the inside to
minimize unwanted scatter. It is tapered and provides as much sparing as
possible for the bodily parts such as the nose, cheeks, and eyebrows of
the subject 50 and has three optical apertures. The outside finishing
matches that of the scanning laser ophthalmoscope 52 to which it is fitted
tightly as a clip-on, easy to remove if necessary. All three apertures are
protected by anti-reflection coated glass, centered on the optical axis,
and may contain an optical filter as specified below. The aperture that is
closest to the scanning laser ophthalmoscope measures about 5 by 5 cm, and
is parallel with the slanted window of the scanning laser ophthalmoscope
54 at 15.6 degrees. This is helpful to eliminate unwanted reflections. The
front aperture, about 1.5 by 1.5 cm, is facing the subject's eye,
vertically, at a comfortable distance as to avoid touching the eyelashes.
The third aperture, 2 by 2 cm, is superior or inferior, slanted, and faces
a CCD monochrome videocamera with objective 56, which is firmly attached
to the scanning laser ophthalmoscope. An appropriate objective would be
the 25 mm 1.4 C mount Cosmicar lens from Asahi Precisicn Co., LTD., Japan
with the 5 mm extension tube. An appropriate CCD videocamera would be the
Sony XC-75 equipped with a 1/2 inch size interline-transfer CCD. The IR
blocking filter has been removed from the camera and replaced by a dummy
glass. The CCD camera can be easily removed and is in a fixed position
relative to the encasement, even if the scanning laser ophthalmoscope
moves. The axis of the CCD camera is vertical and coincides with the
optical axis. An adjustable bite-bar 58 is attached to the movable
chinrest 60 of the instrument. The scanning laser ophthalmoscope with the
attachments described can move vertically with the help of a software
controlled stepper motor 62. Two other stepper motors realize the
horizontal movements of the scanning laser ophthalmoscope on a platform
64. A computer, equipped with one or two OFG cards, mouse, and keyboard
control is provided 66, 68. One TV monitor displays the eye fundus with
graphics overlay using the scanning laser ophthalmoscope. Another TV
monitor or channel displays the anterior segment of the eye focussed on
the iris with the CCD camera 70. It provides the exact position of the
optical entrance pupil of the scanning laser ophthalmoscope as an overlay.
The monitors, luminous control buttons, and reading light for the examiner
are optically isolated from the subject. The only light reaching the
subject is coming from the reduced-in-size anterior window of the scanning
laser ophthalmoscope. This reduction is size is provided by a simple black
diaphragm. It reduces unwanted light from within the scanning laser
ophthalmoscope, unmodulated parts of the laser raster, and the horizontal
edges of the video-fields that are produced by the horizontal scanning.
These edges often have a higher irradiance because of the inertia in the
scanning galvanometer mirror.
OPTICAL PATHWAYS IN THE MODIFIED SCANNING LASER OPHTHALMOSCOPE
With a modification and extension of the scanning laser ophthalmoscope
optics it is possible to [1] visualize the posterior pole, retina or
fundus of one eye in detail; [2] to project graphics, for example a 8 by 8
pixel square that is brighter than the background in Maxwellian view onto
the retina; [3] simultaneously view the anterior segment of the eye on the
same or different monitor, focussed on the iris plane and unambiguously
demonstrating with the help of overlays the exact position of the laser
beam in Maxwellian view used to draw the background and stimulus onto the
retina; [4] both observations do not interfere with each other, the
quality of the retinal image does not imply a lower quality of the iris
image and the observation of the iris does not interfere with the purpose
of psychophysical testing, for example by introducing a bright background;
and [5] it is possible to independently of each other move the stimulus or
graphics on the retina to any desired position and simultaneously, at
will, move the scanning laser ophthalmoscope and entrance pupil of the
laser beam within the anatomical pupil of the iris. Thereby the point of
entry in the pupillary plane can be a variable in psychophysical testing
where the stimulus location is kept constant and also it is possible to
keep the entrance pupil fixed while moving the stimulus position on the
retina.
In general, this is realized by [1] introducing a beamsplitter to provide
two optical paths, one for the scanning laser ophthalmoscope proper and
the second for visualizing the anterior segment of the eye; [2] The
introduction of a separate illumination source for the anterior segment
with appropriate filters to block unwanted light from reaching the CCD
camera. [3] Active or passive stabilization of the stimulus on the retinal
image, and entrance pupil of the Maxwellian view system. [4] The use of
overlays with an appropriate graphics and videocard for demonstrating the
exact position of the stimulus on the retina, and the exact position of
the laserbeam in the anatomical pupil, these parts of the laserbee being
used to create the background and stimulus visible to the subject. [5] The
ability to move, manually or with the help of stepper motors the scanning
laser ophthalmoscope relative to the subject, and the stimulus position on
the retina with the help of the mouse or computer program.
Movement of the scanning laser ophthalmoscope in a frontal plane will cause
the rays of the incident laser beam to use a different portion of the
entrance pupil. This should not cause a shift in the position of the
retinal image because the movement will displace the scanning laser beam
rays in parallel and parallel rays will be focussed on the same spot. This
same spot will however be illuminated in an oblique fashion and therefore
a Tyndall phenomenon can be observed on the monitor, equivalent to the
classic dark field microscopy. This is somewhat similar to the use of a
central stop instead of a pinhole at the retinal conjugate plane.
The separate illumination source for the anterior segment can be a
superluminescent light emitting diode LED, a close relative of the laser
IR diode. The SY-IR53L is a Gallium Aluminium Arsenide super-high output
infrared emitting diode in a T-13/4 package. It produces noncoherent,
nonpolarized IR energy at 880 nm. 880 nm produces negligible interference
with the psychophysical testing. The dispersion angle at half power point
is 20 degrees. This is important to insure a fairly homogeneous
illumination of the anterior segment from a short distance. The forward
voltage is typically 1.3 V, power dissipation is 20 mA. Therefore a
rechargeable NiCd battery pack is ideal to provide several hours of
uninterrupted service. The radiant power output is at least 3.4 mW/sq.cm,
enough to illuminate the entire anterior segment with a single diode. The
small package allows a flexible montage of the diode in the encasement.
The illumination of the anterior segment and anatomical pupil does not
interfere with the fundus imaging. The wide angle distribution of the
light, angulation of the diode, and a partial IR barrier filter 72 with
manual control of the diaphragm in the objective reduce the interference
from a first Purkinje image of both infrared lightsources. In particular
the IR barrier filter removes most of the convergent 780 nm light during
the testing procedure. An example is the Kodak Wratten gelatin filter 87C
with no transmittance for 632.8 nm, 0.5% for 780 nm and more than 90% for
880 nm. Another example is the Kodak Wratten gelatin filter 87 with no
transmittance for 632.8 nm, 30% transmittance for 780 nm and more than 90%
transmittance for 880 nm. Two or more copies of this filter can be in
series. This filter is only removed during calibration of the instrument
when it becomes necessary to align the pivot area of the Maxwellian beam
of 632.8 nm with an overlay on a piece of paper. This calibration is
usually very steady since the beamsplitter, CCD camera and scanning laser
ophthalmoscope are fixed relative to each other. The CCD camera is
optimally sensitive for IR illumination. The crisp image of the anterior
segment with the iris and pupil serve as the fiducial landmark for the
manual or automated localization of the entrance pupil of the scanning
laser ophthalmoscope. It should be stressed that the pivot point of 632.8
nm does not always correspond with the pivot point of 780 nm, and
lightscatter will make the pivot point area look much larger than it is in
reality, especially if higher energies are used. Only a specific portion
of the 632.8 nm beam is used for creating the stimulus and will therefore
determine the true point of entry of the laserbeam. This may be different
from the average position of the total beam. In practice it can be useful
to distinguish the 4 quadrants of the pivot area. Further subdivision is
not practical since the entrance beam has a very complex shape and the
psychophysical test results have a relatively large margin of variability,
exceeding the gain in accuracy from subdividing the entrance beam.
In general, the beamsplitter should favor transmission of 632.8 nm and 780
nm. Especially the transmission of 780 nm is critical for obtaining good
fundus images. As discussed before the 780 nm can be replaced with a diode
laser source of longer wavelength. The choice of the dielectric coatings
for the beamsplitter 46 depends on the wavelengths used, their
polarization status, and the angle of incidence of the laserlight. The
ideal beamsplitter is nearly non-absorbing and as such the reflectance
will be independent of the angle of incidence of the laserlight. The
beamsplitter 46 consists of a single plane-parallel glassplate, 5 by 5 cm,
with a partially reflecting low absorption dielectric coating on one side.
The other side has an antireflection coating optimized for the angle of
incidence of 45 degrees. This will prevent ghost images appearing on the
video display monitor. A sample coating is HEBBAR.TM. (Melles-Griot,
Irvine, Calif.). Some beamsplitters are highly polarization sensitive. In
a particular scanning laser ophthalmoscope, the infra-red laser has
vertical polarization of the E vector, p-plane with regard to the
beamsplitter 46. The He-Ne laser has horizontal polarization of the E
vector, s-plane with regard to the beamsplitter 46. Often ideal
transmission-reflection characteristics can not be realized for all
wavelengths involved. The most optimal coating permits a maximum of 780 nm
to transmit, a maximum of 632.8 nm to transmit, and most of the 880 nm to
reflect. The Melles-Griot, Irvine, Calif. #BTF 001 passes about 50% of the
s-polarized 632.8 nm He-Ne light, more than 90% of the p-polarized 780 nm
diode IR, and reflects about 20% of the 880 nm mixed polarization IR LED.
Two options exist for equalizing within the laser raster the distribution
of irradiant power of 632.8 nm light. As explained before, small
differences in intensity are proper to the scanning laser ophthalmoscope
for various reasons. The acousto-optic modulator can adjust intensities
according to the location where stimuli are presented or the differences
in irradiance can be neutralized with a custom build graded neutral
density filter 74.
The insertion of an optical filter 74 and beamsplitter 46 in the optical
pathway reduces the power of the 780 nm laserlight that illuminates the
retina and additionally reduces the amount of light collected by the
photodetector from the retina. It is reasonable to use a laser source that
is more powerful. This source is already available in the scanning laser
ophthalmoscope for indocyanin green angiography and is easily adapted for
use with the modified scanning laser ophthalmoscope.
FUNDUS AND PUPIL TRACKING WITH THE MODIFIED SCANNING LASER OPHTHALMOSCOPE
As mentioned before, the entrance pupil and stimulus location on the retina
can be selected independently from each other and are subject to a passive
or active feedback mechanism to ensure their position regardless of eye
movements or fixation shifts. The passive mechanisms include a bite-bar 58
and chinrest 60 to maintain the eye position. A simple trial-error-repeat
strategy is used for presenting a stimulus to one specific retinal
location under visual feedback control. Typically the pivot area is
focussed on the iris using 632.8 nm light, prior to testing. The distance
between iris and scanning laser ophthalmoscope is then maintained
throughout the testing procedure in a passive fashion.
The active mechanisms make use of digital image processing techniques for
pupil and fundus tracking. A second OFG board 68 within the same 486/33
mHz can perform these tasks using for example a technique called
two-dimensional normalized gray-scale correlation. Such software is
provided by Imaging Technology, Inc, Bedford, Mass. A 7 degree rotational
tolerance and 120 ms search time are acceptable for the rather liberal
requirements in clinical testing procedures. For pupil tracking the
reference picture is the anterior segment, for retina tracking it is the
fundus image. Feedback regarding eye movements and fixation shifts can be
used to adjust the stepper motors 62, 64 or the psychophysical test
algorithm. Typically the anterior segment image is either up-down or
right-left reversed. It is possible to multiplex both video coming from
the scanning laser ophthalmoscope and CCD camera. Therefore one computer
with one boards is sufficient.
SUMMARY, RAMIFICATIONS, AND SCOPE
The modified scanning laser ophthalmoscope is an electro-optical device
that broadens the range of clinical applications of the conventional
scanning laser ophthalmoscope. With the device it is possible to visualize
simultaneously the anterior segment of the eye with the exact position of
the scanning laser beam, in Maxwellian view, and the posterior segment of
the eye, the retina with graphics. Both observations are independent and
do not influence each other in any adverse way. The observations occur in
real-time. Small areas on the retina can be studied unambiguously with the
scanning laser ophthalmoscope using tests that require knowledge of the
position of the entrance pupil. In imaging, precise positioning of the
entry pupil facilitates normal and off-axis dark-field viewing of the
retina and allows measuring dark-adapted thresholds, Stiles-Crawford I and
II effects, the Campbell effect, and dark-adaptation constants for
specific areas of the fundus that can be visualized on the monitor. Active
feedback tracking algorithms are the final step towards truly automated
static microperimetry strategies. Although the description above contains
many specificities, these should not be construed as limiting the scope of
the invention but as merely providing illustrations of some of the
presently preferred embodiments of this invention. Other embodiments of
the invention including additions, subtractions, deletions, or
modifications of the disclosed embodiment will be obvious to those skilled
in the art and are within the scope of the following claims. The scope of
the invention should be determined by the appended claims and their legal
equivalents, rather than by the examples given.
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